A surgical adhesive can have several biomedical applications. For instance, a surgical adhesive can be used as a replacement of suture, as a surgical sealant to prevent air and fluid leaks, or as a drug delivery reservoir for the delivery of bioactive compound. The primary function of surgical adhesives to hold two pieces of tissues together with a strong adhesive bond, which will last through the healing process. After the healing process, the adhesive will ideally disintegrate into nontoxic products, which are then eliminated from the body. A surgical adhesive should ideally have good handling properties, set up quickly in moist environment with adequate bond strength. In addition it should be nontoxic, biocompatible and biodegradable.
Many different types of tissue adhesives have been reported in the medical and materials literature. Cynoacrylates and fibrin based adhesive systems have been useful. Cynoacrylate based adhesives are excellent tissue adhesives but the toxicity of cynoacrylate monomer and concern over its toxic degradation products has effectively prevented it from getting regulatory approval. Fibrin glue is a biological adhesive derived from human or animal blood. Fibrin based adhesives are commercially available in Europe under the trade name Tissucol® and Tissel® and have been recently approved for use in USA. Typical commercial fibrin glue kit consists of a vial of lyophilized concentrated pooled blood human fibrinogen that also contains fibronectin, Factor XIII and reduced amounts of plasminogen. The concentrate is reconstituted with a reconstituting solution and warmed to 37° C. The second component of the adhesive system is a lyophilized bovine thrombin solution, which is also reconstituted with calcium chloride solution. The formulation may also contain additional components like fibrionolysis inhibitor. The reconstituted solutions are mixed and used as a surgical adhesive system. Fibrin adhesives have been demonstrated to be nontoxic, biocompatible and bioresorbable.
The mechanism of fibrin glue involves last stages of coagulation cascade, in which fibrinogen is converted to fibrin in presence of thrombin, Factor XIII, fibronectin and ionized calcium (Ca+2). The speed of this coagulation process depends on the thrombin concentration used and may be varied according to the need. The resultant fibrin clot or gel is primarily held up by electrostatic and hydrogen bonding and is susceptible to rapid dissolution by proteolytic enzymes such as plasmin. Factor XIII via transamination introduces the covalent crosslinks, which makes the fibrin clot resistant to proteolytic degradation. It also improves the mechanical properties of fibrin glue.
Fibrinogen is the third largest abundant protein component of blood plasma and is perhaps most important component of fibrin adhesive formulation. Fibrinogen is the third largest abundant protein component of blood plasma and is perhaps most important component of fibrin adhesive formulation. A second important protein is Factor XIII whose concentration is 0.015 mg/ml and has a molecular weight 320 KD. In order to have good adhesive properties and fast gelation times, a higher concentration of fibrinogen is desired in a coagulable protein concentrate. The strength of adhesive bond formed is directly proportional to concentration of fibrinogen in the formulation and method of its preparation. Cryoprecipitation is the most common method used in preparation of coagulable protein concentrate. This method involves a) freezing a fresh blood plasma which has been screened for hepatitis or AIDS at −80° C. for at least 6 hours preferably for at least 12 h. b) raising the temperature of frozen plasma to around 0-4° C. so as to form a supernatant and a cryoprecipitated suspension containing fibrinogen and Factor XIII and c) recovering the cryoprecipitated suspension by decanting the supernatant. Another method described in the patent and medical literature is the use of common low toxic organic/inorganic compounds such as ethanol, polyethylene glycol, poly(vinyl alcohol), 1-6-hexanoic acid, ammonium sulfate and glycerol.
Most of the methods reported in the literature have one common feature, which is isolation by phase separation or precipitation step. All the precipitation approaches suggested for the preparation of the fibrinogen-containing fraction for this purpose are too time consuming and complex to be finished in a short time period to be accomplished during the course of the surgery. Also in some approaches such as cryoprecipitation, special equipment like refrigerated centrifuges are required. Different methods of precipitations produce fractions with different adhesive characteristics. Also different methods of precipitation produce precipitates of different particle size. Some finer particle sizes are difficult to separate from supernatant liquids, which may result into poor yield of final protein concentrate. Many times multiple precipitation and redissolution steps are required to achieve desirable concentrations. Many methods rely on preparation in an open test tube systems. These open test tube products are often stored for extended periods of time in refrigerator and methods that may not meet the requirements of the American Association of Blood Banks for open-system storage of blood products. Phase separation by precipitation may also denature the protein and alter its natural conformation. Many enzymatic reactions are sensitive to protein conformation. Isolation by precipitation may also affect the yield of final product. Many times up 10-20% coagulable protein is lost in such processes.
The bovine thrombin used in commercial and autologus fibrin adhesive formulation may carry bovine spongioform encephalitis (BSE, responsible for mad cow disease) and other viruses pathogenic to humans. Also, bovine thrombin is a potent antigen, which can cause immunological reactions in humans. Thus, the use of bovine thrombin involves the potential risks to the patient.
The fibrin clot formed using commercial fibrin glue formulations is degraded by the proteolytic enzymes found inside the human body. The current formulations do not provide any control over its degradation.